1. Field of the Invention
The present invention generally relates to an aberration measuring apparatus of an optical system, and more particularly, to an aberration measuring apparatus which measures wavefront aberration of a projection optical system of a photolithography apparatus which transfers a pattern on a mask to a photosensitive substrate. Such a photolithography apparatus is used, for example, in a lithography step when a semiconductor device is manufactured.
2. Related Background Art
When a micro semiconductor device such as a semiconductor memory or logic circuit is manufactured using a photolithography (printing) technology, a projection photolithography apparatus is conventionally used which transfers a circuit pattern drawn on a mask or reticule (these are used as mutually interchangeable terms in the present application) by projecting it onto a wafer, etc., through a projection optical system.
A minimum transferable size (resolution) through a projection photolithography apparatus is proportional to the wavelength of light used for exposure and inversely proportional to the numerical aperture (NA) of a projection optical system. Therefore, the resolution increases as the wavelength is shortened. For this reason, in response to a growing demand for miniaturization of semiconductor devices in recent years, light of shorter and shorter wavelengths is used for exposure light sources such as an ultra-high pressure mercury lamp (i-line) (wavelength: approximately 365 nm), KrF excimer laser (wavelength: approximately 248 nm) and ArF excimer laser (wavelength: approximately 193 nm). In the future, an F2 laser (wavelength: approximately 157 nm) and EUV light (Extreme Ultra Violet) (wavelength: approximately 13 nm) are seen as promising alternatives.
The projection photolithography apparatus is required to accurately transfer a pattern on a mask to a wafer at a predetermined magnification ratio (reduction ratio) and in order to meet such a demand, it is important to use a projection optical system with excellent image formation performance with aberration reduced to a minimum. With reference to FIG. 13 to FIG. 15, the measuring principles for measuring wavefront aberration of the optical system used for wavelengths of 193 nm or below will be explained. FIG. 13 is a schematic block diagram showing an example of a conventional aberration measuring apparatus 1000.
The aberration measuring apparatus 1000 uses an excimer laser 1100 as the light source for an interferometer as in the case of an exposure light source of an optical system PO (projection optical system) to be analyzed. However, the excimer laser 1100 has an intensity pattern which is an oblong rectangular pattern to narrow the spectral band of emitted light beams and has low spatial coherence for reasons related to the structure of a resonator, and therefore the excimer laser 1100 alone is not adequate as the light source for an interferometer. Therefore, a wavefront shaping unit 1110 for shaping wavefront of light emitted from the excimer laser 1100 is provided to alleviate these conditions.
The wavefront shaping unit 1110 is constructed of a toric optical system for transforming a rectangular pattern into a square pattern, a lens for condensing the light beam transformed into the square pattern, a lens for restoring the spread light beams after condensing to parallel light beams and a spatial filter placed close to such a condensing point.
“The light beam whose intensity pattern has been shaped by the wavefront shaping unit 1110 and whose spatial coherence has been improved is branched into two optical paths by a half mirror 1200. The light beam which has passed through the half mirror 1200 passes through an objective lens 1210 and an optical system PO to be analyzed and is reflected by a spherical mirror 1220. The point of image formation of the optical system PO matches the curvature center of the spherical mirror 1220. For this reason, the light beam reflected by the spherical mirror 1220 propagates on the same optical path in the reverse direction, is reflected by the half mirror 1200, passes through a first pupil image formation lens 1310 and a second pupil image formation lens 1320 and enters a CCD camera 1400 as light to be analyzed.”
On the other hand, the optical path of the light beam reflected by the half mirror 1200 is turned up by a turnup mirror 1510, the light beam is then reflected by a reference mirror 1520, returns on the same optical path and passes through the half mirror 1200. The light beam which has passed through the half mirror 1200 is introduced into the CCD camera 1400 as reference light by the first pupil image formation lens 1310 and second pupil image formation lens 1320.
Through interference between these two light beams (that is, the light to be analyzed and the reference light), an interference pattern is detected by the CCD camera 1400. To calculate wavefront aberration of the optical system PO to be analyzed from the detected interference pattern, a so-called pattern scanning method is used. This method calculates an initial phase of the interference pattern from a plurality of interference pattern images when a length of optical path difference between the light to be analyzed and the reference light is scanned. Such a length of optical path difference is scanned by synchronizing the reference mirror 1520 with the CCD camera 1400 by a control section 1600.
Here, when the wavefront aberration of the optical system PO to be analyzed is measured, it is necessary to control the wavelength of the light source during measurement. This is because the optical system PO to be analyzed includes chromatic aberration, and thus the measurement result varies depending on the wavelength of the light source during measurement. Therefore, the control section 1600 acquires the wavelength at the time of measurement from a wavelength measuring mechanism 1120 provided in the excimer laser 1100. The wavelength measuring mechanism 1120 is calibrated by an etalon, etc., and the measuring accuracy of its absolute value is guaranteed by calibrating it using an absorption line such as platinum. Furthermore, when the light source is an F2 laser, it is normally not provided with the wavelength measuring mechanism 1120 and therefore calculated values or values measured using a spectroscope, etc., are used.
Furthermore, as shown in FIG. 14, by replacing the objective lens 1210 with a TS (Transmit Sphere) lens 1710 and replacing the spherical mirror 1220 with an RS (Reflective Sphere) mirror 1720, it is possible to use the light beam reflected by a final plane (that is, TS plane) 1710a of the TS lens 1710 as reference light.
The principle of the aberration measuring apparatus using an excimer laser light source has been described so far. However, when an excimer laser is used as a light source for an interferometer of an aberration measuring apparatus used to measure wavefront aberration of a projection optical system for a wavelength of 193 nm or less for which no excellent continuous wave light source exists as the light source for an interferometer, the projection optical system contains a chromatic aberration, and the same light source as that for the photolithography apparatus is used through the aberration measuring apparatus used must measure wavelengths with high accuracy, and therefore the measuring accuracy of wavelengths is not sufficient. Moreover, since its coherence length is short, it is actually very difficult to construct an interferometer which requires a length of optical path difference represented by a Fizeau interferometer as shown in FIG. 14.
Furthermore, when an excimer laser is used as the light source for an interferometer, its spatial coherence, time coherence and directional stability are insufficient, and therefore if a spatial filter for improving the spatial coherence is placed, the stability of beam amount after passing through the spatial filter deteriorates due to low directional stability and the measuring accuracy of wavefront deteriorates. Furthermore, a thick spectral line width which causes low time coherence provokes overlapping of interference patterns by different wavefronts due to chromatic aberration of the optical system to be analyzed, and therefore the contrast deteriorates and it is difficult to realize high accuracy measurement. Moreover, the low time coherence is sensitively reflected in a reduction of contrast due to a deviation in the length of the optical length path difference, and therefore the arrangement of the optical system needs to be changed when measuring wavefront errors caused by a system which is indispensable for high accuracy measurement of an optical system to be analyzed, that is, measurement of system errors, and exact measurement of a system error is impossible.
Furthermore, there is also a problem that the contrast of interference patterns decreases due to a decrease in transmittance of the optical system PO to be analyzed caused by shortening of wavelengths for photolithography apparatuses in recent years. The decrease in the contrast of interference patterns increases measurement errors of wavefront aberration due to disturbance such as variations in a beam amount of laser from the light source.
FIG. 15 is a graph showing a variation in the contrast of interference patterns due to transmittance of the optical system PO to be analyzed. A dashed line indicates an interference pattern when the transmittance of the optical system PO to be analyzed is 100% and the contrast at this time is 100%. On the other hand, a solid line indicates an. interference pattern when the transmittance of the optical system PO to be analyzed is 50% on one way. Since the beam amount of the reference light is equal to that of the light to be analyzed, the contrast of interference pattern deteriorates to approximately 80%.
Furthermore, when both the TS lens and RS mirror have a low reflective index, the beam amount of the light source necessary for the CCD camera to obtain an appropriate beam amount increases. That is, it is not possible to use a light source which is unable to obtain a sufficient beam amount output as the interferometer, and attempting to obtain a sufficient beam amount causes an enormous load on the light source and a pulse light source using-wavelength transformation deteriorates long-term maintainability of the light source such as deterioration of crystals and optical devices, etc.